Electrolyte for an aluminium-electrolyte capacitor, capacitor which uses the electrolyte and the use of said capacitor

ABSTRACT

The invention concerns an operating electrolyte for an electrolytic capacitor that also includes, in addition to ethylene glycol, 1 to 10 weight percent of a derivatized aromatic carboxylic acid. The invention also concerns an aluminum electrolytic capacitor with the operating electrolyte according to the invention. In addition, the invention concerns the use of the capacitor according to the invention. The operating electrolyte according to the invention is distinguished by an improved sparking voltage and permits the capacitor according to the invention to be used at voltages up to 530 V.

[0001] The invention concerns an operating electrolyte for an aluminumelectrolytic capacitor that contains ethylene glycol. In addition, theinvention concerns an aluminum electrolytic capacitor with the operatingelectrolyte. The invention also concerns the use of the aluminumelectrolytic capacitor.

[0002] Aluminum electrolytic capacitors are constructed as a stack oflayers of a cathode foil, which may be, for example, a thin, roughenedAl foil having a thickness between 20 and 50 μm, and of an anode foil,which may be, for example, a roughened Al foil around 100 μm thick, andwhich has a dielectrically operative oxide layer that is deposited onthe foil by an electrochemical process. The cathode foil too has a thinoxide layer, around 1.5-3 nm thick. The stack of layers is usuallypresent in the form of a coil wound around a mandrel, which is builtinto an aluminum cup.

[0003] Between the foils there is a course of one or more layers ofpaper, which is impregnated with the operating electrolyte. Thisoperating electrolyte constitutes the actual cathode.

[0004] Operating electrolytes of the type named at the beginning thatalso contain ammonium benzoate are known. German Patent 693 04 732 Part2 discloses an electrolyte that contains benzoic acid or its ammoniumsalts and additional aliphatic dicarboxylic or tricarboxylic acidstogether with solvents, which include ethylene glycol and otheralcohols. These operating electrolytes have the disadvantage of a lowsparking voltage of about 300 V. Consequently, the aluminum electrolyticcapacitors made with the known operating electrolytes are not suitablefor operation with higher voltages.

[0005] The object of the present invention is therefore to provide anoperating electrolyte for aluminum electrolytic capacitors that has ahigh sparking voltage.

[0006] This object is achieved according to the invention by anoperating electrolyte according to claim 1. Additional embodiments ofthe invention, an aluminum electrolytic capacitor with the operatingelectrolyte according to the invention, and the use of the aluminumelectrolytic capacitor, may be seen from the additional claims.

[0007] The invention specifies an operating electrolyte with:

[0008] a) 70-99 weight percent ethylene glycol

[0009] b) 1-20 weight percent of a substance that contains derivatizedaromatic carboxylic acids.

[0010] In addition to these essential components, the operatingelectrolyte according to the invention may contain small quantities ofadditional components that do not detract from the desired properties ofthe electrolyte.

[0011] The operating electrolyte according to the invention has theadvantage that it is possible to achieve a higher sparking voltage byexchanging the ammonium benzoate for a substance containing derivatizedaromatic carboxylic acids.

[0012] Furthermore, despite the increased sparking voltage, the changein conductivity of the operating electrolyte is insignificant.

[0013] In addition, an operating electrolyte for aluminum electrolyticcapacitors that contain the following components is particularlyadvantageous:

[0014] a) 90-99 weight percent ethylene glycol

[0015] b) 1-10 weight percent of a substance that contains derivatizedaromatic carboxylic acids.

[0016] In addition to these essential components, the operatingelectrolyte according to the invention may also contain small amounts ofother common components that do not detract from the desired propertiesof the electrolyte.

[0017] In addition, it is particularly advantageous to select thederivatized aromatic carboxylic acids from a group of substances thatinclude substituted cinnamic acids and ammoniumtrans-4-phenyl-3-butenoate.

[0018] Cinnamic acids that can be considered for substitution hereinclude in particular ammonium cinnamate, ammonium o-methylcinnamate,ammonium p-methylcinnamate, ammonium α-methylcinnamate and ammonium4-hydroxycinnamate.

[0019] The invention also specifies an aluminum electrolytic capacitorin which the electrolyte is the operating electrolyte according to theinvention. The capacitor has a stack of layers, with an anode layer anda cathode layer. Positioned between the anode layer and the cathodelayer is a separating layer impregnated with the electrolyte. The anodelayer and the cathode layer each have an oxide layer on their side thatfaces the separating layer. The anode layer and the cathode layer are ofaluminum, while the oxide layers are of aluminum oxide.

[0020] The aluminum electrolytic capacitor according to the inventionhas the advantage that it can be operated at a higher operating voltagedue to the use of the operating electrolyte with high sparking voltageaccording to the invention.

[0021] The invention also specifies the use of the aluminum electrolyticcapacitor according to the invention at an operating voltage between 100and 530 V. Particularly when using ammonium trans-4-phenyl-3-butenoateas derivatized aromatic carboxylic acid, use of the capacitor accordingto the invention at an operating voltage of up to 530 V is conceivable.

[0022] The invention is explained in greater detail below on the basisof exemplary embodiments and the associated FIGURE.

[0023] The FIGURE shows as an example the capacitor coil of an aluminumelectrolytic capacitor according to the invention, in schematic crosssection.

[0024] The capacitor coil of the electrolytic capacitor according to theinvention consists of an anode layer 1, which has an anode oxide layer 4on its top side. Anode layer 1 is an aluminum foil with a thickness ofabout 50-120 μm. Anode oxide layer 4 consists of aluminum oxide, and hasa thickness of between 0.05 and 2 μm.

[0025] Positioned above anode oxide layer 4 is a separating layer 3.Separating layer 3 has a thickness between 30 and 200 μm, and consistspreferably of paper. Positioned above separating layer 3 is a cathodelayer 2, which has a cathode oxide layer 5 on its side facing towardseparating layer 3. Separating layer 3 is impregnated with the operatingelectrolyte according to the invention.

[0026] Cathode layer 2 is made of aluminum, and has a thickness between20 and 60 μm. Cathode oxide layer 5 consists of aluminum oxide and has athickness between 1.5 and 3 nm. Anode layer 1 and cathode layer 2 eachhave electrically conductive connections to a capacitor connector 6.

[0027] The stack of layers shown in the FIGURE normally is in the formof a coil wound around a mandrel, which is built into an aluminum cup.

[0028] In the following Table 1, six different exemplary embodiments ofan operating electrolyte according the invention (B2 through B7) arecompared to a known operating electrolyte (B1) in regard to theircomposition, their conductivity and their sparking voltage. TABLE 1Composition of various operating electrolytes B1 B2 B3 B4 B5 B6 B7Ethylene glycol 96 96 96 96 96 96 96 [weight %] Ammonium benzoate 4 0 00 0 0 0 [weight %] Ammonium cinnamate 0 4 0 0 0 0 0 [weight %] Ammoniumo-methyl- 0 0 4 0 0 0 0 cinnamate [weight %] Ammonium p-methyl- 0 0 0 40 0 0 cinnamate [weight %] Ammonium α-methyl 0 0 0 0 4 0 0 cinnamate[weight %] Ammonium 4-hydroxy 0 0 0 0 0 4 0 cinnamate [weight %]Ammonium trans-4- 0 0 0 0 0 0 4 phenyl-3-butenoat [weight %]Conductivity 1.4 1.1 1.1 1.1 1.1 1 1 [mS/cm] Spark voltage 300 525 120510 460 420 530 [V]

[0029] The exemplary embodiments show that the sparking voltage can beincreased substantially in some cases by replacing ammonium benzoatewith a derivatized aromatic carboxylic acid. At the same time, theconductivity changes only slightly; it is reduced somewhat compared tothe known operating electrolyte (B1).

[0030] The operating electrolytes according to B2, B4 and B7 inparticular show a significantly increased sparking voltage and permit analuminum electrolytic capacitor made with them to be used atcorresponding elevated operating voltages.

[0031] The invention is not confined to the embodiments shown asexamples, but is defined in its most general form by claim 1 and claim5.

1. Working electrolyte for an aluminum electrolytic capacitor with: (a)70-99 wt. % of ethylene glycol and (b) 1-20 wt. % of a substance whichis selected from a group which comprises the following carboxylic acidsand carboxylic acid salts: substituted cinnamic acids, ammoniumcinnamate, ammonium α-methyl cinnamate, and ammoniumtrans-4-phenyl-3-butenate.
 2. Working electrolyte according to claim 1with: 90-99 wt. % of component (a) and 1-10 wt. % of component (b). 3.Working electrolyte according to one of claims 1-2, in which thesubstituted cinnamic acids comprise ammonium-o-methyl cinnamate,ammonium-p-methyl cinnamate, and ammonium 4-hydroxycinnamate. 4.Aluminum electrolytic capacitor with a working electrolyte according toone of claims 1-3, which capacitor has a capacitor winding with an anodelayer (1), a cathode layer (2), and a dielectric layer (3) between thefirst two layers, this dielectric layer being saturated with the workingelectrolyte, and in which both the anode layer (1) and the cathode layer(2) have an oxide layer (4, 5) on the side facing the dielectric layer(3).
 5. Use of an aluminum electrolytic capacitor according to claim 4for working voltages of 100-530 V.